The invention relates generally to measuring the temperature of heated metal surfaces without contacting the same, and particularly to a method of correcting inherent errors in temperature readings due to changes in the emissivity of heated aluminum surfaces by a novel calibration procedure that compensates for the temperature errors due to changes in emissivity.
A perfect radiator or black body is characterized by the fact that the energy which it emits depends only on the absolute temperature of the body. A non-black body or grey body radiator emits then only a fraction of the energy emitted by a perfect radiator, the fraction being dependent on the emissivity of the body. Thus, in order to relate energy emitted by a non-black body to true temperature, its emissivity must be compensated for.
The emissivity of an opaque body is expressed as the ratio of radiated energy emitted from a near black body to the energy emitted from a black body, both at the same absolute temperature.
Emissivity is also related to incident radiation that is reflected by the body by the equation E+R=1, i.e., when the emissivity is unity, the reflectivity is 0, and the emitted energy is directly related to the true body surface temperature. However, non-black bodies, such as heated aluminum surfaces, are partial reflectors of radiation, such that their emissivity is always less than unity. Hence, the total energy leaving an area of such a body will, in general, not be directly related to true surface temperature unless its emissivity is determined and an appropriate correction applied.
An aluminum alloy object is particularly troublesome, as the emissivity of its surfaces often changes from one discreet area to the next. Similarly, the emissivities of a plurality of equal temperature objects can change from object to object. One primary reason for this lies in the varying roughness of the surfaces. Another reason lies in varying alloy constituents of the materials of the objects, which constituents oxidize in varying degrees, depending on such parameters as the temperature value to which the objects are exposed and length of time the objects are maintained at the temperature value. Also, the length of time the objects are exposed to the atmosphere before heating affects emissivity, as well as the constituency of the atmosphere itself.
Further, any dust and dirt in the atmosphere and in furnaces employed to heat the objects, and any substances applied to the objects in the course of the manufacture thereof, such as lubricating oils and/or cooling emulsions, affect the emissivity of the surfaces. One furnace in a plant, for example, will be relatively clean and therefore heat metal ingots, without depositing contaminants thereon while another furnace in the same plant will be relatively dirty and hence produce ingot surfaces that have received and contain such dirt.
For these reasons, heated surfaces offer varying degrees of emissivity such that determining their true temperature with radiation thermometers is unreliable, this being true for all processes, including rolling, forging, casting and extruding, and the products thereof, such as metal plate and sheet, forgings, castings and extrusions.
Commercially available, single wavelength, narrow band radiation thermometers provide an output signal that is a linear function of the infrared energy radiated from heated surfaces. All such thermometers are calibrated from a black body source (E.congruent.1.0). Therefore, unless a surface has a characteristic black body value, the single wavelength thermometer indicates a relative temperature value that is lower than the absolute surface temperature of the body. The difference between the true temperature of the surface and that of the lower relative temperature value indicated by a single radiation thermometer is a function of surface emissivity; the lower the surface emissivity, the lower the measured temperature value.
In the case of commercial two-color or dual wavelength ratio radiation thermometers, a linearized output reading as a function of temperature can be made to behave in a manner opposite to the single wavelength thermometer, i.e., the lower the surface emissivity, the higher the apparent temperature reading, as compared to the black body calibration value. An electric circuit capable of ratioing output signals of two detectors is used, thereby cancelling emissivity measurement error if surface emissivity ratio values are equal or constant at both selected wavelengths, which is the case at grey body conditions. For this reason, the dual wavelength ratioing thermometer provides a substantially more accurate indication of true surface temperature than the single wavelength technique, especially for grey body or near-grey body surfaces.